Using the detailed vulnerabilities to create a related exploit is "ridiculously simple," and can be initiated "without any notification or user confirmation," Valotta said in a blog post. "All you need is to type one key on Internet Explorer or make one click on Google Chrome" for the exploit to succeed.

The detailed vulnerabilities begin with modern browsers using modeless notifications to alert users to such events as file downloads, plug-in installations or authorizing HTML5 privileged APIs. "These notifications bars are non-invasive, they are designed ... to inform users without interrupting navigation, but they suffer from some serious design problems," Valotta said.

Notably, such notifications appear only in the related window or tab that generated them. "So if you are able to 'hide' the navigation window, you can hide also the notification, this means you can [as an example] download a file on your computer and have no notification at all from your browser," he said. In addition, even though these windows might be hidden, the notification is active, meaning that the screen will accept a keyboard shortcut -- for example to save a file or run a file.

Here's Valotta's attack scenario: A user visits a malicious website, which opens a pop-under window and begins downloading a malicious file. Pop-under windows -- they appear behind the active window -- can be created using JavaScript, for example via the cross-browser tool js-popunder, which is available for free from GitHub.

Getting that malicious file to execute, however, requires a bit of trickery, which is also known as social engineering. With Internet Explorer, for example, a user would have to be tricked into typing a required key -- "r" on English-language systems, to make a downloaded file run, or "e" on Italian systems, and so on -- which would in fact be transmitting the keystroke to the hidden, but active, pop-under window.

How might this be accomplished? "Well, there are plenty of ways: a game, a typing lesson," said Valotta. "But my favorite one is a Captcha. Just take a fake Captcha starting with the proper letter ("r" or "e") and you'll get 100% of tricked users."

While a letter must be typed to complete the exploit via IE, for Chrome browsers the requirement -- after the file has been downloaded -- is different. "You need to trick the victim into clicking on some link/button on the foreground window," said Valotta. "The attacker, using some [JavaScript], is able to track mouse pointer coordinates so -- as soon the mouse is hovering on the button -- the attacker can close the foreground window." Obviously, attackers would need to get their timing right.